The present invention is related to a bit stream reproducing apparatus used to transfer audio data with employment of a digital leased line.
Since digital signal processing techniques are greatly advanced in which after either analog video signals or analog audio signals have been quantization-coded, signal process operations are carried out, it is easily possible to perform signal process operations of analog information. In particular, since information amounts of audio signals are small, as compared with those of video signals, signal process amounts of the audio signals per unit time required to decode the quantization-coded audio signals so as to obtain original analog audio signals are small. Therefore, these quantization-coded audio signals may be processed by way of digital signal processors and the like.
To effectively transfer information contained in digital video/audio signals, the entire digital video/audio data are subdivided into preselected amounts of digital video/audio data which will then be transferred. Moreover, since this information is compressed, the data transfer in the low rate can be realized. In particular, as to audio data, the data compressing/expanding methods such as "Moving Picture Experts Group (will be referred to as an "MPEG" hereinafter)" established in ISO/IEC SC29/WG where the data compressing/expanding method is standardized by ISO/IEC 11172-3, and this rule is standardized have been widely used in audio transfers, digital satellite broadcasting systems, and DVD with employment of digital leased lines.
As one example of bit stream data, there is an MPEG audio bit stream in which audio data which has been processed by a quantization-coding process operation and thereafter by a compression-coding process operation, quantization-coding information indicative of a quantization-coding method for this audio data, and also compression-coding information indicative of a compression-coding method for the audio data are subdivided into a plurality of packet data, and synchronization data representative of a starting position thereof is added thereto.
FIG. 7A is a diagram for representing a data structure of an MPEG audio bit stream. Audio Access Units (will be referred to as an "AAU" hereinafter) 100 are transferred one by one in a time sequential manner in unit of packet data reproducible as audio data. In the case of MPEG 1 audio layer 1, assuming now that a sampling frequency is selected to be Fs(KHz), AAU is defined as follows: EQU AAU(1 frame)=(384.times.bit rate)/Fs[bit].
In the case of MPEG1 audio layer 2, AAU is defined as follows: EQU AAU(1 frame)=(1125.times.bit rate)/Fs[bit].
As represented in FIG. 7B, the AAU 100 of this MPEG1 audio layer 1 is arranged by a 32-bit header 101, a 16-bit CRC 102, audio data 103 whose length is variable by a bit rate, and ancillary data 104. In the case of MPEG1 audio layer 1, as indicated in FIG. 7C, the audio data 103 is arranged by bit allocation information 106, a scale factor 107, and audio sample data (0 to 11) 14D.
Also, in the case of MPEG1 audio layer 2, as indicated in FIG. 8, the audio data 103 is arranged by bit allocation information 106, scale factor selection information 120, a scale factor 107, and audio sample data (0 to 11) 140.
Furthermore, the header 101 indicated in FIG. 7C is arranged by a 12-bit syncword 109, various information ID 110, a layer 111, a protection bit 112, a bit rate index 113, a sampling frequency 114, and the like, as shown in FIG. 7D.
Normally, in order that not only bit stream data, but also the MPEG audio bit stream are detected to be dataprocessed, the following process operation is carried out.
That is, the header 101 is provided at a head of detection data, into which a sync signal having a specific value is inserted, and when this sync signal is detected, the data detection is commenced.
The 16-bit data CRC 102 shown in FIG. 7C corresponds to error checking data. Since this data CRC is decoded, a quality of transferred data (to check whether or not there is a failure, e.g., data changes occurred in data transfer operation) can be checked.
FIG. 9 is a block diagram for indicating a conventional bit stream reproducing apparatus for the MPEG audio bit stream. In this drawing, reference numeral 201 shows a syncword detector, reference numeral 220 indicates a buffer, and reference numeral 202 denotes a CRC checking circuit. Also, reference numeral 203 represents a bit stream dividing device, reference numeral 204 shows a side information decoder, reference numeral 205 indicates a dequantizing device, reference numeral 206 is a denormalizing device, and reference numeral 207 denotes a sub-band combiner. The buffer 220, and the above-described circuits 202 to 207 constitute a bit stream decoder 210.
Next, a signal flow will now be simply explained.
The MPEG audio bit stream is inputted to the syncword detector 201 by which the syncword 109 is detected. In the case that the syncword 109 is detected, since the data starting position of the AAU 100 is determined, the data subsequent to the syncword 109 contained in the header 101 are detected.
Next, after all of the information contained in the header 101 have been detected by the syncword detector 201, a predetermined amount of data is stored into the buffer 220. The data outputted from the buffer 220 is CRC-checked by the CRC checking circuit 202. It should be noted that since the CRC data is optional in the MPEG audio bit stream, only when the protection bit 112 contained in the header 101 is equal to "1", the CRC data 102 is added.
The data outputted from the CRC checking circuit 202 is entered into the bit stream dividing device 203. This bit stream dividing device 203 divides the inputted data into each of information units such as bit allocation information 106, scale factor 107, and audio sample data 140. In particular, both the bit allocation information 106 and the scale factor 107 are outputted to the side information decoder 204.
In the side information decoder 204, both the bit allocation information 106 and the scale factor 107 which correspond to the side information are decoded to produce bit numbers, namely bit allocation data for determining a data length of each of audio samples, and also a scale factor corresponding to a coefficient value used in a calculation for this bit allocation. These bit numbers are allocated to each of the audio samples during the dequantizing operation. Then, these bit numbers and scale factors are sent out to the dequantizing device 205 and the denormalizing device 206.
In the dequantizing device 205, the audio data samples 140 are dequantized based upon the output data (namely, information such as quantizing bit number) from the side information decoder 204. The denormalizing device 206 multiplies the output derived from the dequantizing device 205 by the scale factors entered from the side information decoder 204. Since the MPEG audio data is divided into 32 audio subdata by the highspeed Fourier transform during the coding operation, the calculation results of the denormalizing device 206 are recombined with each other by the sub-band combiner 207 to thereby produce the original digital audio signal.
Since the conventional bit stream reproducing apparatus is arranged in the above-described manner, in such a case that the bit allocation information 106 corresponding to partial data contained in the inputted MPEG bit stream signal could not be correctly transferred due to occurrences of the transfer noise, the data length of the subsequently transferred audio sample data 140 and the data starting position of the audio sample data to be dequantized are erroneously recognized. As a result, there is a problem that acoustic noise may occur.
Also, in such a case that the inputted MPEG bit stream signal could not be correctly transferred due to noise, for instance, when such an erroneous detection is made that the syncword 109 indicative of the data starting position is now located at a position after the normal position of the syncword 109, a data length of one frame would be erroneously recognized as a data length longer than the actual data length. Since this implies that the next frame information is reproduced as the present frame information, the frame information would be dropped out.
Further, in the case that either a portion or an entire portion of the scale factor information corresponding to partial data contained in the inputted MPEG bit stream could not be correctly transferred, since the calculation in the denormalizing device 206 corresponding to the signal process operation at the post stage could not be carried out, there is another problem that acoustic noise may be produced.